In recent years, there has been a rapid advancement of portable and cordless electronic devices for consumer use such as telephones, personal computers and video cameras. With this advancement, demand is growing for small and light-weight secondary batteries having a high energy density as a power source for driving these devices. Particularly, the development of non-aqueous electrolyte secondary batteries is vigorously conducted. In a non-aqueous electrolyte secondary battery, a lithium-containing composite oxide is used as a positive electrode active material. As a negative electrode material, a carbon material capable of absorbing and desorbing lithium ions, silicon compound or tin compound is used. Between positive and negative electrodes is interposed generally a separator comprising a microporous film composed of polyethylene (hereinafter referred to as “PE”), polypropylene (hereinafter referred to as “PP”) or the like. As a non-aqueous electrolyte, an aprotic organic solvent containing a lithium salt such as LiBF4 or LiPF6 dissolved therein is used.
With an increase in CPU speed for portable personal computers, the amount of heat generated therein is increasing. Moreover, demand is strong for devices with a longer operation time. Under the circumstances, as a power source for such devices, demand is also growing for non-aqueous electrolyte secondary batteries capable of being charged to a higher voltage so as to increase capacity. However, when a non-aqueous electrolyte secondary battery is charged to a charge voltage of not less than 4.2 V in a high temperature environment, the battery performance deteriorates significantly.
From the viewpoint of preventing such deterioration, technical development for separators is conducted vigorously. As the separator, a microporous film made of PE having a relatively low melting point is mainly used.
A microporous film made of PE swiftly melts and deforms to electrically cut off the current between the positive and negative electrodes in the event where the battery reaches an overheated state, serving to ensure safety of the battery. However, when a battery is charged to a voltage of not less than 4.2 V, more specifically, when the positive electrode potential is not less than 4.2 V relative to the potential at which metal lithium is dissolved and deposited, the PE separator is oxidized, which might generate gas.
Patent Document 1: Japanese Laid-Open Patent Publication No. 2001-273880